The present invention generally relates to mobile radiocommunication systems.
In mobile radiocommunication systems, there is a general requirement for higher performances, including higher quality of service and/or higher capacities. At the same time there is also a general requirement for smaller, cheaper and less power consuming equipments, in particular user equipments (or mobile stations).
Mobile radiocommunication systems generally use multiple access techniques to enable a plurality of users to share a given bandwidth, by allocating different channels to these different users.
In CDMA (Code Division Multiple Access) systems, different channels correspond to different spreading codes, each spreading code enabling to spread a signal from a basic rate to a higher rate (or chip rate), according to the known principles of spread-spectrum techniques.
As is known, CDMA systems are interference-limited systems. That means, if the interference level increases too much, then the performances (in terms of quality and/or in terms of capacity) may decrease to non acceptable values. Therefore there is a need to reduce interferences in such systems, to improve performances.
Generally, interference can be reduced by using spreading codes which are as far as possible orthogonal to each other. However, even by choosing orthogonal spreading codes, interference may still be generated when a receiver receives a received signal which is the superposition of different signals (hereinafter also called path signals) corresponding to different propagation paths of a same transmitted signal. Such different path signals may be created either non intentionaly, by reflection on various elements of the environment, or intentionaly, by use of diversity transmission techniques, i.e. by transmitting simultaneously from different network equipments, or base stations, to a same mobile station.
In CDMA systems, contrary to other multiple access systems such as for example TDMA (Time Division Multiple Access) systems, such multi-path propagation can be exploited in a receiver to improve the quality of the detected signal.
For example, in a so-called Rake receiver, different fingers or paths are provided, each finger being intended to process a given path signal, and the results provided by the different fingers then being combined in an optimal way to optimize the quality of the detected signal.
However, the signal applied to each finger is the superposition of the different path signals corresponding to the different propagation paths, and therefore the processing of a given path signal still suffers from the interference from all other path signals.
This problem is even more serious for receivers in mobile stations, i.e. for the downlink transmission direction from network to mobile stations, for the following reasons.
Besides user information intended to be transmitted between users, a mobile radiocommunication system generally requires, for the system to operate properly, that the network broadcasts some common information to all mobile stations, such as in particular for enabling the mobile stations to get synchronised with the network. A signal transmitted in downlink is therefore the superposition of different signals (hereinafter also called channel signals), corresponding to different channels (or physical channels) carrying these different types of information, and generally called, respectively, downlink physical dedicated channels and downlink physical common channels.
For example, in wideband CDMA systems like UMTS (xe2x80x9cUniversal Mobile Telecommunication Systemxe2x80x9d) such downlink common physical channels may include:
Physical synchronization channels (PSCH), for the mobile stations to perform frequency and/or time synchronization. In UMTS, there are two synchronization channels: primary SCH to perform slot synchronization and secondary SCH to perform frame synchronization and to identify the scrambling code group of the downlink scrambling code of the cell.
Primary common control physical channel (P-CCPCH), carrying the broadcast channel (BCH) that gives system information (number of the cell, Random access channel (RACH) scrambling code(s), . . . ).
Secondary common control physical channel (S-CCPCH), carrying the paging channel (PCH) and the forward access channel (FACH).
Primary and secondary common pilot channel (P-CPICH and S-CPICH), used for several purposes like channel estimation or handover.
Since the downlink common channels must be received by all mobile stations (or MSs) in a cell, their transmit power must be pretty large, e.g. 10% of the total transmit power of a base transceiver station (or BTS) and, moreover, they are not power controlled (i.e., their transmit power is constant).
Therefore, when a MS is close to a BTS, the interference caused on the downlink dedicated channel signal(s) of this MS, for a given path, by the downlink common channel signals from all other paths, is significant. Thus, to keep the same quality, i.e. to keep the downlink signal-to-interference ratio (SIR) close to a given target SIR, the MS will ask the BTS, via a power control loop mechanism generally provided in these systems, to increase the transmit power of its dedicated channel signal(s). This will cause an additional increase of interference for other MSs that will in turn require, via this power control loop mechanism, an increase of the BTS transmit power. For a given total transmit power this will therefore result in a loss of downlink system capacity. Thus, such interference caused by downlink common channel signals should as far as possible be avoided, or at least reduced, to improve the performance of the system.
Specific processing techniques, also called interference cancellation techniques, have been proposed for this purpose.
WO 97/08846 discloses a method for substractive multiple access interference cancellation, wherein a wireless subscriber unit receives multiple forward link signals and estimates the data being transmitted via each forward link signal. In response to these estimates an associated ideal waveform is generated for each forward link signal received. For each forward link signal processed, the ideal waveform of the other forward link signals are substracted from the signal level of that forward link signal being processed before the data carried is determined. In an alternative embodiment, the estimation of data is performed on a single channel, or sub-set of channels, carried by the forward link signals, with at least the signal level associated with a pilot channel being estimated.
EP 0 876 002 discloses a CDMA receiver which receives and demodulates a coherent CDMA signal including at least one user data channel and a separate pilot channel received over a plurality of L paths, where the desired data channel is orthogonal to the pilot channel for a given path. The CDMA receiver comprises L path demodulators, each demodulator for estimating a data channel and a pilot channel from a CDMA signal received over one of the L paths and for generating L-1 cancellation signals each to be used by a specific one of L substractor means. Each of the L substractor means is used for substracting the L-1 cancellation signals, produced by different ones of the other L-1 path demodulators, from the CDMA signal associated with substractor means. In an embodiment, the L-1 cancellation signals are reconstructed pilot signals, and each of the substractor means is located prior to its associated demodulator to substract the reconstructed pilot signals from the signal inputted to its demodulator.
Therefore these two references correspond to a same solution, wherein in each finger of the Rake receiver, the corresponding path signal of the pilot signal (or even, in the case of the first reference, the corresponding path signal of the total received signal) is estimated, and substracted from each other path signal in each other finger, prior to processing by each of these other fingers.
This solution is illustrated in FIG. 1, in a receiver classicaly including: an antenna 1, a Rake receiver 2, and some intermediate processing 3 (including RF processing, automatic gain control, reception filtering, . . . etc.) between the antenna 1 and the Rake receiver 2. The Rake receiver 2 classicaly comprises L fingers 41, 42, . . . 4L and a combiner 5, each finger being controlled in a known way by means 6 for estimating the channel impulse response of the transmission channel, which determine the amplitudes, phases, and delays, of the different propagation paths.
In this solution, interference cancellation requires L estimators 71, 72, . . . 7L for carrying out the above-mentioned estimations, and L(L-1) substractors 812, . . . 81L, 821, . . . 82L, . . . 8L1, 8L2, . . . for carrying out the above-mentioned substractions.
A drawback of this solution is that it requires a lot of signal processing and calculations, and therefore significantly increases the complexity and the power consumption of the equipments, which is not desirable, in particular for user equipments (or mobile stations).
Therefore there is a need to avoid such a drawback, while still providing an efficient interference cancellation.
Besides, in the first of the above-mentioned references, such an interference cancellation is always carried out, i.e. even if the pilot signal does not significantly interfere the useful signal (which will be the case if the mobile station is not close to a base station). This has the drawback that interference cancellation is needlessly performed in this case, which also needlessly increases power consumption of the equipments.
In the second of the above-mentioned references, the pilot signal cancellation can be switched on and off, based on a minimum mean-squared error criterion, to provide minimum mean-squared error Rake output. However such a criterion has the drawback that it involves a lot of very complex processing and calculations.
Therefore, there is also a need to avoid such a drawback, while still providing an efficient criterion to perform or not such an interference cancellation.
An object of the present invention is a receiver for a CDMA mobile radiocommunication system, receiving a signal which is a superposition of different signals, or path signals, corresponding to different propagation paths of a transmitted signal, which in turn is a superposition of different signals, or channel signals, corresponding to different channels carrying different transmitted informations, said receiver comprising, prior to the recovering of the transmitted information carried by a given channel, by exploiting said different propagation paths:
means for estimating at least one received channel signal, or interferer signal, which is a superposition of different path signals corresponding to different propagation paths of a transmitted channel signal corresponding to at least one channel different of said given channel,
means for substracting said at least one estimated interferer signal, from said received signal.
According to another object of the invention, said given channel is a downlink dedicated channel and said at least one channel different of said given channel is a downlink common channel.
According to another object of the invention, said means for estimating said at least one interferer signal include:
means for estimating the channel impulse response h(t) of the transmission channel, said means providing an estimate ĥ(t),
means for computing c(t)*ĥ(t), where c(t) is the transmitted channel signal corresponding to said at least one channel different of said given channel, and where * is the convolution operator, said means providing an estimate c(t)*ĥ(t) of said interferer signal.
According to another object of the invention, said means for estimating said at least one interferer signal include:
means for estimating the channel impulse response h(t) of the transmission channel, said means providing an estimate ĥ(t),
means for estimating the transmitted channel signal c(t) corresponding to said at least one channel different of said given channel, said means providing an estimate ĉ(t),
means for computing ĉ(t)*ĥ(t), where * is the convolution operator, said means providing an estimate ĉ(t)*ĥ(t) of said interferer signal.
According to another object, said receiver further comprises:
means for controlling said means for substracting, as a function of the power of said estimated interferer signal.
According to another object, said means for controlling comprise:
means for determining the power P of said at least one estimated interferer signal,
means for enabling said substraction if the following criterion is valid: P greater than xcex1.I
where xcex1 is a predetermined parameter, and I is a reference power.
Another object of the present invention is an equipment for a CDMA mobile radiocommunication system, including a receiver of the above kind.
Another object of the present invention is a user equipment for a CDMA mobile radiocommunication system, including a receiver of the above kind.